An optical fiber link (a link utilizing an optical fiber) has been widely used in common house holds, particularly for music-related purposes. The optical fiber link is used for communicating optical digital signals in digital audio systems such as CD (compact disc) players, MD (mini disc) players, DVD (digital versatile disc) players, and amplifiers having a digital input terminal. In each of the above listed digital audio systems, a light receiving/emitting device for use in the optical fiber link is adopted for inputting/outputting the optical digital signals. Specifically, a light emitting device (optical transmitter) for use in the optical fiber link is used to output the optical digital signals, and a light receiving device for use in the optical fiber link is used to input the optical digital signals.
In recent years, the optical fiber link has been also widely used for communicating music signals in portable systems such as laptop computers, portable phones, and MPEG3 (Motion Picture Experts Group)-1 Audio Layer 3) players. Under these circumstances, it is required that power consumption be reduced in the light receiving/emitting device, so that batteries of these portable devices last longer.
Since the optical fiber is a signal communicating medium which is light weight and excellent in noise-resistance characteristics, the optical fiber link is also suitable for communicating signals in in-vehicle systems. Examples of currently available in-vehicle-use optical fiber links for practical applications are MOST (Media Oriented Systems Transport) and IDB (ITS Data Bus)1394. The low current consumption is required also in the light receiving/emitting device for use in the optical fiber link in a vehicle, so that the battery of the vehicle lasts longer.
Typically, the light emitting device (optical transmitter) includes an LED and an LED driving circuit for driving the LED.
Various types of LED driving circuits are known. One is a differential drive type LED driving circuit disclosed in Japanese Laid-Open Patent Publication No. 242522/1998 (Tokukaihei 10-242522; published on Sep. 11, 1998). Another is a single drive type LED driving circuit disclosed in Japanese Laid-Open Patent Publication No. 4202/2000 (Tokukai 2000-4202; published on Jan. 7, 2000). In FIGS. 10 and 11 shown are typical conventional LED driving circuits for use in the optical fiber link.
A circuit shown in FIG. 10 is a differential drive type LED driving circuit, suitable for high-speed LED driving. An input electronic signal IN (voltage Va) is inputted into an inverter INV1 having an N-channel MOS (Metal Oxide Semiconductor) transistor and a P-channel MOS transistor. An output signal from the inverter INV1 is inputted into a differential amplification circuit having (i) N-channel MOS transistors MN1 and MN2 and (ii) a constant current source for supplying a constant driving current Imod. More specifically, the output signal from the INV1 is inputted to a gate of the N-channel MOS transistor MN1. Further, the output signal from the INV1 is inverted by an inverter INV2, and is inputted to a gate of the N-channel MOS transistor MN2. An LED is connected between a drain of the N-channel MOS transistor MN2 and a power source line (power source voltage Vcc). When a voltage Vb of the output signal from the inverter INV1 is low level, and a voltage Vc of the output signal from the INV2 is high level; that is, when the input electronic signal IN is high level, the N-channel MOS transistor MN1 is turned OFF, and the N-channel MN2 is turned ON. As a result, the driving current Imod flows in the LED, causing the LED to emit light. On the contrary, When the voltage Vb of the output signal from the inverter INV1 is high level, and the voltage Vc of the output signal from the INV2 is low level (i.e. when the input electronic signal IN is low level), the N-channel MOS transistor MN1 is turned ON, and the N-channel MOS transistor MN2 is turned OFF. As a result, the driving current stops flowing in the LED, causing the LED to stop emitting light. Further, if a current Iled flowing in the LED is completely brought down to zero while the LED is not emitting light, flowing the driving current Imod in the LED do not immediately leads to emission. In short, a delayed light emission will occur. In order to reduce such a delay, a bias current Ibias is usually supplied to the LED while the LED is OFF. The bias current Ibias is supplied within a predetermined range of an extinction ratio (ON/OFF ratio in the luminous intensity of the LED). Further, in order to achieve high-speed LED driving, a peaking current Ipeak for peaking the driving current is often flown through the LED at the rise and fall of the LED driving current Imod (waveforms of the LED driving current is reshaped to have a peaking point at the rising and falling edges)(See FIG. 12). As shown in FIG. 10, the output signal (Voltage Vb) from the inverter INV1 is inverted by the inverter INV3, and by a capacitor Cp and a resistor Rp, the peaking current Ipeak is generated from the inverted signal. It should be noted that FIG. 12 shows the waveforms (time-dependent change) of (i) the voltage Va of the input electronic signal IN, (ii) the voltage Vb of the output signal from the inverter INV1, (iii) the voltage Vc of the output signal from the inverter INV2, (iv) the voltage Vd of the output signal from the inverter INV3, (v) the driving current Imod, the peaking current Ipeak, (vi) the current Iled flowing through the LED, and (vii) output light (luminous intensity).
In the differential drive type LED driving circuit shown in FIG. 10, the driving current Imod constantly flows through a differential circuit (N-channel MOS transistors MN1 and MN2). This causes a difficulty in reducing current consumption. Further, due to a delay in an inverter INV3, the peaking current lags behind the driving current. This causes the driving current and the peaking current to rise at different timings. Since the shift in rising timing causes the peak of the current flowing in the LED to appear some time after the rise of the current, the rising time and falling time becomes longer. This may cause distortion in a pulse width, which, in turn, may cause difficulties in accelerating the response speed of the LED emitting light.
A circuit shown in FIG. 11 is a conventional example of the single drive type LED driving circuit. An input signal IN is inputted to an inverter INV1, inverted by the inverter INV2, and then inputted to a gate of an N-channel MOS transistor MN3. A current from a constant current source Io is inputted to a current mirror circuit having an N-channel MOS transistor MN1 and an N-channel MOS transistor MN2. In the case where the N-channel MOS transistors MN1 and MN2 have the same gate length, a current flowing through the N-channel MOS transistors MN1 and a current flowing in the N-channel MOS transistor MN2 are proportional to the gate widths of the respective N-channel MOS transistors MN1 and MN2. Usually, the current Io is set to 1/N (where N is usually 2 or more) of the driving current Imod, so that the current flowing into the drain of the N-channel MOS transistor MN2 becomes the driving current Imod when N is the gate width ratio of the N-channel MOS transistor MN2 to the N-channel MOS transistor MN1 (i.e. gate width of MN2/gate width of MN1). The drain of the N-channel MOS transistor MN2 is connected with a source of the N-channel MOS transistor MN3, and an LED is connected between the drain of the MN3 and a power source terminal Vcc. When the gate of the N-channel MOS transistor MN3 is at high level, the N-channel MOS transistor MN3 is turned ON. As a result, the driving current Imod flows into the LED, causing the LED to emit light. On the contrary, when the gate of the N-channel MOS transistor MN3 is at low level, the N-channel MOS transistor MN3 is turned OFF, thereby causing the LED to stop emitting light. Here, while the N-channel MOS transistor MN3 is turned OFF, the current does not flow into the N-channel MOS transistor MN2. Accordingly, unlike the differential driving type shown in FIG. 10, there will be no constant flow of the driving current Imod. This allows for reduction of current consumption.
However, in the single driving type LED driving circuit shown in FIG. 11, the peaking current is determined based on a size of the N-channel MOS transistor MN3 for use in switching. This causes difficulties in controlling a peaking amount (height of a rising peak), which, in turn, may cause difficulties in accelerating the response speed of the LED emitting light.
As described, the differential drive type LED driving circuit shown in FIG. 10 is suitable for high speed driving; however, a constant flow of the driving current in the differential circuit causes difficulties in reducing current consumption. Further, it is difficult to match the ON/OFF timing of the driving current with the ON/OFF timing of the peaking current. Therefore, it is difficult to accelerate the response speed of the LED emitting light.
On the other hand, the single drive type LED driving circuit shown in FIG. 11 is advantageous in reducing current consumption, because the driving current does not flow while the LED is turned OFF; however, it is difficult to freely control the peaking amount, because the peaking current value is determined based on the size of the MOS transistor for use in switching, i.e. the N-channel MOS transistor MN3. Further, as shown in FIG. 13, the driving current is peaked while the LED is ON, but the driving current is not peaked while the LED is OFF. This extends the falling time of light from the LED. Thus, it is difficult to accelerate the response speed of the LED emitting light.